The present invention relates to a polyimide film having a denatured surface and a process for denaturing the polyimide film. Particularly, the invention relates to a polyimide film comprising aromatic polyimide prepared from an aromatic tetracarboxylic acid component which comprises 3,4,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic acid, its anhydride or its ester and an aromatic diamine component which comprises p-phenylenediamine, containing a micro-granular filler of metal atom-containing inorganic material, and having a surface so denatured that the denatured surface has increased adhesiveness.
Aromatic polyimide having been prepared from an aromatic tetracarboxylic acid component and an aromatic diamine component by polymerization and dehydration shows a high heat resistance as well as a high mechanical strength and a high chemical stability. Therefore, the aromatic polyimide is widely employed in various industrial field, such as fields of art for preparing devices utilizable in electronics and outer space.
The aromatic polyimide has excellent characteristics in various properties, but disadvantageously shows poor adhesiveness when it is prepared in the form of a film. An aromatic polyimide film having poor adhesiveness is not easily bonded to other films such as an aromatic polyimide film or a metal film such as a copper foil with a conventional adhesive. Heretofore, various studies have been made for providing adhesives for firmly bonding an aromatic polyimide film to a-metal foil. Most of the known adhesives are still unsatisfactory for firmly bonding an aromatic polyimide film to a metal foil.
In order to bond firmly an aromatic polyimide film to a metal foil, treatment of a surface of an aromatic polyimide film with corona discharge or plasma discharge processing has been also studied to increase the adhesiveness of a surface of an aromatic polyimide film.
Japanese Patent Provisional Publication No. 63-61030 describes a polyimide film in which an oxygen/carbon ratio on its surface is increased by 0.01 to 0.1, and an amount of volatile material in the film is made to 0.45 wt. % or less, by corona discharge processing.
Japanese Patent Provisional Publication No. H1-138242 describes that an aromatic polyimide film is denatured in its surface by discharge processing such as corona discharge processing under the condition that the aromatic polyimide film is kept on a discharge electrode.
Japanese Patent Provisional Publication No. H3-56541 describes a process for denaturing an aromatic polyimide film by treating the film with an organic solvent and then processing the film by plasma discharge.
Japanese Patent Provisional Publication No. H5-1160 describes a process for denaturing an aromatic polyimide film by discharge processing such as plasma discharge processing using a specifically designed electrode at an atmospheric pressure under the condition that the aromatic polyimide film is kept on a drum electrode.
In the above-mentioned publications, it is stated that the described denaturing process is favorably applied to various heat-resistant resin films such as aromatic polyimide films. In the working examples of the publications, however, the corona discharge processing or plasma discharge processing is applied only to a film of aromatic:polyimide which is prepared from pyromellitic dianhydride and 4,4-diaminodiphenyl ether (commercially available in the name of xe2x80x9cKaptonxe2x80x9d), for increasing adhesiveness of the polyimide film.
According to the studies by the present inventors, however, it cannot be said that all aromatic polyimide films can be increased in the adhesiveness on their surfaces by the known corona discharge processing or plasma discharge processing. For instance, there is known a film of aromatic polyimide which is prepared from an aromatic tetracarboxylic acid component comprising 3,4,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic acid, its anhydride or its ester and an aromatic diamine component comprising p-phenylenediamine. An aromatic polyimide film of this type has a higher heat-resistance, a higher mechanical strength and/or better chemical stability, as compared with the above-mentioned film of aromatic polyimide prepared from pyromellitic dianhydride and 4,4-diaminodiphenyl ether, but has a disadvantageous feature in that the film is hardly increased in its surface adhesiveness, due to, possibly, its high chemical stability. According to the studies by the present inventors, the film of aromatic polyimide prepared from an aromatic tetracarboxylic acid component comprising 3,4,3xe2x80x24xe2x80x2-biphenyltetracarboxylic acid, its anhydride or its ester and an aromatic diamine component comprising p-phenylenediamine cannot be denatured to give a satisfactory adhesiveness by the conventional discharge processing (particularly, plasma discharge processing at an atmospheric condition).
Accordingly, it is an object of the present invention to provide a film of aromatic polyimide prepared from an aromatic tetracarboxylic acid component comprising 3,4,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic acid, its anhydride or its ester and an aromatic diamine component comprising p-phenylenediamine whose surface has good surface activity, particularly, increased adhesiveness.
The present invention resides in a polyimide film of 20 to 125 xcexcm thick which comprises aromatic polyimide prepared from an aromatic tetracarboxylic acid component comprising 3,4,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic acid, its anhydride or its ester and an aromatic diamine component comprising p-phenylenediamine and contains a micro-granular filler of metal atom(e.g., Si, Ca, Mg)-containing inorganic material, in which at least one surface of the polyimide film has been so processed by plasma discharge that an amount of the metal atom and a ratio of oxygen/carbon increase by 0.03 to 1.0 atomic % and 0.01 to 0.20, respectively, on the processed surface. The increases of the metal atom amount and the oxygen/carbon ratio are calculated based on the metal atom amount and the oxygen/carbon ratio on the untreated polyimide film.
The above-mentioned polyimide film of the invention can be easily prepared by a process for denaturing a polyimide film of 20 to 125 gm thick on its surfaces, the polyimide film comprising aromatic polyimide prepared from an aromatic tetracarboxylic acid component which comprises 3,4,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic acid, its anhydride or its ester and an aromatic diamine component which comprises p-phenylenediamine and containing a micro-granular filler of metal atom-containing inorganic material, which comprises processing the polyimide film on its surfaces by plasma discharge, by running the polyimide film in a gaseous mixture of argon and hydrogen between a plurality of activated plasma discharge electrodes arranged in a double line under such condition that the running polyimide film is free from contact with the electrodes.
Preferably, the surface of the polyimide film of the invention has been so processed by plasma discharge that the amount of the metal atom and the ratio of oxygen/carbon increase by 0.03 to 0.92 atomic 6 (more preferably 0.03 to .0.8 atomic %, most preferably 0.03 to 0.5 atomic %, specifically preferably 0.05 to 0.5 atomic %) and 0.01 to 0.20, respectively, on the processed surface. Preferably, the surface of the polyimide film of the invention has been so processed by plasma discharge that the amount of the metal atom is in the range of 0.5 to 1.5 atomic %, and the ratio of oxygen/carbon increases by 0.11 to 0.15, on the processed surface.
The micro-granular filler of metal atom-containing inorganic material is preferably selected from the group consisting of particles of silicon dioxide, titanium dioxide, magnesium oxide, calcium carbonate, and talc having a mean particle size in the range of 0.01 to 5.0 xcexcm. The micro-granular filler is preferably contained in the polyimide film in an amount of 0.02 to 5.0 weight %, based on the amount of the polyimide film.